1. Field of the Invention
The invention relates to a process for working up residues in the preparation of carboxylic acids, in particular the preparation of dicarboxylic acids having from 8 to 16 carbon atoms.
2. Discussion of the Background
The oxidation of, for example, cyclododecanol and cyclododecanone to dodecane-1,12-dioic acid by means of nitric acid in the presence of vanadium catalyst has been described many times in the literature (cf., for example, FR 1331267 and DE 22 33 590). The by-products of this synthesis are mainly dibasic acids having from 4 to 12 carbon atoms, also known as degradation acids, smaller amounts of monobasic acids and nitrogen-containing compounds, and are obtained in one stream after isolation of the main product, dodecanedioic acid. These by-products, which further comprise the vanadium catalyst and other optional metal catalysts such as copper compounds, are formed in small amounts, and can under certain conditions contaminate biological wastewater treatment processes.
The German patent application DE 16 68 564 describes a process in which a by-product stream comprising degradation acids is worked up, and the catalysts, in this case copper and vanadium, are recovered. For this purpose, particular ratios of degradation acids, nitric acid and inorganic salts are set, and the relatively long-chain degradation acids are separated off as an oily phase, while the aqueous nitric acid together with the copper and vanadium catalysts and the short-chain degradation acids are recirculated back into to the process. However, the type of separation method described in DE 16 68 564 is only possible because relatively large amounts of copper salts (>5%) are present in the process and readily phase separate. The is explicitly described in DE 16 68 564. DE 16 68 564 mentions the possibility of simple phase separation by the addition of inorganic sulfates or nitrates of sodium, magnesium or calcium instead of the copper salts. However, if sodium sulfate, for example, is added and is recirculated to the process after phase separation in a manner similar to that described in DE 16 68 564, the crystallization of, for example, the dodecanedioic acid is adversely affected, and is no longer obtained in a form which meets the required specification. In addition, the addition of salts undesirably increases the complexity of the process, because the concentration of these salts also has to be regulated. The type of process described in DE 16 68 564 thus requires the presence of copper salts or other salts which do not adversely affect the crystallization of the desired product.
According to the French patent FR 13 31 267, a small part of the degradation acids, together with nitric acid, catalyst and water may be recirculated directly back to the oxidation process. The remainder is may be passed to a vaporizer in order to recover the nitric acid. This process has the following disadvantages: the relatively long-chain degradation acids which hinder the crystallization are recirculated back to the process, only a small part of the catalyst can be recovered, and the degradation acids cannot be utilized because they are not isolated.
However, it is particularly desirable to isolate the relatively long-chain degradation acids and utilize them directly, for example, or in the form of derivatives, and to recirculate the vanadium catalyst back to the process. In order to utilize the degradation acids, they must contain only small amounts of nitrate (<1%) and metal catalysts (<100 ppm of metal, in particular vanadium), since both components interfere in the further processing of the degradation acids.
It is therefore an object of the invention to provide a process for separating off and isolating the degradation acids formed in the oxidation, which process does not have the abovementioned disadvantages, makes it possible for at least part of the catalyst to be recirculated and provides degradation acids having a quality which makes their further use possible.